1. Field of the Invention
This invention generally relates to a system for reducing controlled flight into terrain accidents and in particular it relates to terrain data processing and algorithms for terrain awareness and warning systems to prevent controlled flight into terrain accidents.
2. Brief Description of the Prior Art
Controlled Flight into Terrain (CFIT) warning uses the data provided by a flight management computer, Global Positioning System (GPS) receiver and other aircraft data providing systems. The prior art of CFIT warning predicts a three-dimensional flight path based upon a curve-fit extrapolation of the most recent position and velocity data received. This predicted flight path is then compared with the internal terrain map of the immediate area and an assessment of the potential threat of aircraft collision with terrain is computed. The type of systems described above could look up to 120 seconds in advance. This is the so-called xe2x80x9ctime to clearxe2x80x9d in contrast to xe2x80x9ctime before impactxe2x80x9d warning, and also xe2x80x9cterrain avoidancexe2x80x9d in contrast to terrain xe2x80x9cawarenessxe2x80x9d.
The effort to reduce CFIT accidents by U.S. Airlines can be traced back to 1974. The prior art of standard Ground Proximity Warning System (GPWS) uses radio altimeter data to provide an audible warning if an aircraft has insufficient terrain clearance. Flying into precipitous terrain can result in late warnings since the standard GPWS depends upon a downward looking radio altimeter to detect rising terrain.
xe2x80x9cEnhancedxe2x80x9d version of GPWS-EGPWS, which is available from AlliedSignal, and Ground Collision Avoidance System (GCAS), which is available from Sextant Avionique, graphically depict terrain surrounding the aircraft""s flight path on a cockpit display and provide earlier warning. Both systems are built around a three-dimensional terrain database and implement a true predictive look-ahead capability that is based on aircraft climb performance. The technology has three parts: use of GPS and other highly accurate navigation systems to provide precise positioning with updates in seconds; computer technology with greater speed and memory; and accurate, sophisticated worldwide terrain databases including a Digital Terrain Elevation Database (DTED) of the world. FIG. 1 shows schematic system diagram of GPWS, which is from DASSAULT ELECTRONIQUE GCAS.
In EGPWS and GCAS, the terrain database is the system""s core. Looking-ahead algorithms compare the projected future position of the aircraft with this database and warnings are issued on this basis. The using of DTED allows the systems to display terrain in proximity to the aircraft. During the flight, the peaks and terrain below the aircraft in DTED can also be displayed for situation awareness to the pilot. Looking ahead warnings based on comparison between flight profile or flight path and terrain can be used to give an advanced alert. During emergency descents in mountainous areas and during en-route avoidance of inclement weather, looking-ahead warning further helps to prevent any situation that could lead to a CFIT.
However, the terrain database occupies large amounts of memory. If the terrain is mapped at 100-meter intervals over a region 10000Km by 10000Km, there are 1010 grid points. Clearly, the computations inherent in continuously accessing 1010 grid points are formidable and it is necessary to reduce, or compress this information. Besides, many navigation functions can be done with knowledge of terrain, such as allowing for optimum flight plan or emergency change of route in a real time situation, which could be restrained by the amount of data retrieved and the cost of computation when complexities of the algorithms are increased. Another major drawback of DTED files is that it only provides elevation data. Without further processing or aid from other formats of terrain data, such as features or vector representations, DTED gives no geometrical relationship among data elements.
There are many applications, particularly in flight management, where the complete DTED database must be accessed to determine or revise the flight waypoints. The terrain awareness warning and navigation safety related issues described here are performed on an encoded terrain navigation space. The encoding of a grid file of Digital Terrain Elevation Data (DTED) is based on a variant of quad-tree representation of spatial data structures. Each element of a DTED file is encoded in a Morton numbering sequence with respect to its location in the grid file, a scaled elevation data, and coverage of homogeneous (equal elevation) area as features. Under this data structure, a DTED is organized as a set of integer numbers with ascending sequence. Each integer represents a node in which planar location, scaled elevation and coverage are interlaced with a set of position bits to form an integer. The encoded list is defined as the terrain Oct-tree model. Navigation functions not only make reference to terrain Oct-tree for elevation data, but also perform processing and operations on it. The navigation space is transferred from the DTED array to its encoded integer list.
A number of navigation functions are performed on the terrain Oct-tree based navigation space. A preferred embodiment uses a dynamic dangerous zone defined by flight altitude. In the preferred embodiment, a set of nodes of terrain height over a minimum flight altitude are located and aggregated. Algorithms such as collision check, mountainous area boundary and region growing technique are developed as basic operations for this terrain model. Yet another preferred embodiment which takes a visibility graph approach for dynamic route selection has been adopted to reduce the real-time computational requirements. This approach reduces the size of the search space by establishing a partial visibility graph of terrain and avoids details of the terrain, which do not influence the choice of flight path, independent of the size of the navigation space.
Several forms of CFIT warning functions of aircraft navigation become feasible once an aircraft flight path and the topology of a region of terrain can be readily determined. Furthermore, it implies access to a database holding the terrain data in order to initiate the geometric computations. By exploiting the multiple and variable resolution properties of Oct-tree terrain models, a series of CFIT warning functions using terrain data as reference are easily implemented. The functions including Ground Proximity Warning, Obstacle Cueing, Terrain Masking, Perspective Images of Terrain, Passive Ranging, Real-time Route Selection and Route Planning, Weather Display Overlaying, and Waypoint Overlaying.
Prior art planning approaches use pre-defined obstacle models. Moreover, the number of data retrieving DTED and the cost of computation are in the negative. In the present invention, the dangerous zones dynamically vary during the execution of a flight plan. Besides, a layer of Oct-tree terrain is used to facilitate the on-line operation capability.
Other objects and advantages of the present invention will become apparent from the detailed description to follow taken in conjunction with the appended drawings.